Semiconductors are used in integrated circuits for a wide range of applications, including personal computers, music devices, video devices, multimedia devices, digital data devices, communications devices, and so forth. One type of semiconductor device used in a number of such applications is a semiconductor storage device, such as a static random access memory (SRAM), dynamic RAM (DRAM), read-only memory (ROM), flash memory, and so forth.
Semiconductor storage devices allow for the storage of large amounts of data, with ROMs being non-volatile and enabling the storage of information that persists without requiring power. RAMs, on the other hand, allow rapid and random access to information stored therein, but are volatile, requiring power, constant, periodic updates, or both to maintain information integrity.
Semiconductor storage devices are generally arranged in large two-dimensional arrays of individual memory cells. Each row of memory cells may be selected by a row select line (also commonly referred to as a word line) and each column of memory cells may be selected by a column select line (commonly referred to as a bit line). A memory cell at an intersection of an active word line and an active bit line may have information written to it or read from it.
Reading information from a memory cell or writing information to a memory cell may be performed by a sense amplifier. For example, a sense amplifier may read information from a memory cell by detecting a change in a voltage or a current on a bit line coupled to the memory cell (the memory cell being a memory cell located at the intersection of the active word line and bit line).
Since, in general, a wide voltage swing (or wide current swing) may take longer to occur than a small voltage swing (or small current swing), a sense amplifier that may be able to accurately determine the information stored in the memory cell using a small voltage swing is desirable. By being able to utilize a small voltage swing, the sense amplifier may be able to shorten memory access times, thereby improving the performance of the semiconductor storage device. The small voltage swing becomes more critical as the two-dimensional array of memory cells becomes larger with more and more memory cells being attached to each word line and bit line, thereby increasing a capacitive load on the lines. The increased capacitive load slows the voltage swing.
FIG. 1a shows a prior art sense amplifier 100. Sense amplifier 100 makes use of a NAND logic gate 105 to determine the information stored in a memory cell. The use of NAND logic gate 105 may result in a high threshold voltage, often greater than 100 mV (commonly on the order of 300 to 400 mV). This may significantly impact the speed performance of sense amplifier 100, especially as the number of memory cells increase.
FIG. 1b shows a prior art single-ended symmetrical differential sense amplifier 150. Sense amplifier 150 makes use of a cross-coupled latch 155 to determine the information stored in a memory cell. Additionally, sense amplifier 150 includes a pair of transistors (transistor 160 and transistor 162) for use as pass gates, preferably field effect transistors (FET) are used, especially N-type metal oxide semiconductor (NMOS) transistors. Gate terminals of the transistors 160 and 162 may be coupled to an enable signal “PGENB,” which may be used to turn the transistors 160 and 162 on or off.
Transistor 160 may be used to couple or de-couple a bitline “BL” input into cross-coupled latch 155, while transistor may be used to couple or decouple a reference voltage “VREF” into cross-coupled latch 155. The reference voltage is not a readily available voltage, such as VDD, substrate ground, and so forth. Sense amplifier 150 determines the information stored in the memory cell from the voltage of the reference voltage and the voltage on the bit line. However, the use of a reference voltage that is not one of the commonly available voltages may require the use of an additional voltage supply and/or the use of an external voltage supply connected to a semiconductor storage device containing sense amplifier 150.